The heat transfer characteristics of a downhole heat exchanger (DHE) were studied experimentally in a laboratory scale. The permeable aquifer was simulated by using a rectangular tank filled with glass beads of diameter ranging from 3 to 5 mm. The downhole heat exchanger was modeled by a U-shaped copper tube. We have simulated various configurations of DHE by changing the geometrical parameters of U-tube and downhole, and different operating conditions at various inlet water temperatures, mass flow rates through the DHE. In addition, the experimental system was specially designed that capable of simulating an aquifer with flowing water. The results show that the temperature difference between the reservoir and the inlet water of U-tube is a dominant factor on heat extracted. However, the total amount does not necessarily follow a linear relation. The mean outside heat transfer coefficient of the two legs can achieve a maximum value at a moderate temperature difference.

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